You've crashed. On a planet. It's a planet made up of wierd and strange things, creatures that up until your arrival quite likely existed in peace and harmony, and then you brought yourself, your mass of crates which have scattered around the landscape, and the teleporters you so desperately need to get back to your now scuffed up transport ship to get out of dodge. The pixel art approach works well here, and there's a definite consistency and "otherworld" sense that helps sell the experience rather neatly.
Starting the game the first time gives you access to the beginner class - Commando. This class is a good place to learn how to deal with the game, and a balanced set of skills that should see you progress at a reasonable rate. All the unlockable classes are tailored towards specific styles of play, and generally require specialist approaches to get the most out of them. A good example is the enforcer, which is far slower and less mobile than the commando, but due to his riot shield, can essentially soak -ridiculous- quantities of damage from the direction he's facing.
Combined with the precise and well balanced combat, along with the great sense of atmosphere, is potentially one of the best soundtracks of recent times, going alongside things such as Don't Starve, Deus Ex : Human Revolution, and Really Big Sky. A good soundtrack sells the atmosphere, and in this case it's been captured perfectly. The entire package is well worth picking up and playing regardless of if you're new to the genre, or a nethack veteran.